Microporous polyolefin membranes from bespoke solvents

ABSTRACT

Halogen-free, microporous polyolefin membranes are disclosed herein. The halogen-free, microporous polyolefin membranes can be manufactured using an environmentally friendly manufacturing process that includes extrusion of polymer-plasticizer mixtures followed by sheet formation and extraction of the plasticizer with a halogen-free solvent. The halogen-free solvent has a flashpoint greater than about 23° C. and an initial boiling point at least about 50° C. lower than the flashpoint of the plasticizer. The process can further be a closed loop process in which the halogen-free solvent can be reused.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 63/266,830, filed on Jan. 14, 2022, and titled MicroporousPolyolefin Membranes from Bespoke Solvents, which is incorporated hereinby reference in its entirety.

COPYRIGHT NOTICE

© 2023 Amtek Research International LLC. A portion of the disclosure ofthis patent document contains material that is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent document or the patent disclosure,as it appears in the Patent and Trademark Office patent file or records,but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d).

TECHNICAL FIELD

This invention relates to halogen-free, microporous polyolefin membranesthat can be manufactured in an environmentally-friendly, closed loopprocess that includes the extrusion of polymer-plasticizer mixturesfollowed by sheet formation, extraction of the plasticizer with asolvent, evaporation of the solvent to form micropores, and subsequentadsorption-desorption of the solvent from activated carbon for re-use inthe manufacturing process. The bespoke solvent is halogen-free, has lowwater solubility, and a flashpoint above about 23° C. In terms of Hansensolubility parameters, in some instances the solvent has low dispersion(delta D˜15), low polar (delta P˜0), and low hydrogen bonding (deltaH˜0) characteristics such that it is miscible with plasticizers thatinclude naphthenic, paraffinic, and white mineral oils. Finally, therelative boiling points are critical for (1) separation and recycling ofthe plasticizer and bespoke solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting a closed loop solvent extraction andcarbon bed recovery method used in the manufacture of microporousmembranes in accordance with one embodiment.

FIG. 2 is a diagram depicting a closed loop solvent extraction and vaporcondensing recovery method used in the manufacture of microporousmembranes in accordance with another embodiment.

BACKGROUND OF THE INVENTION

Microporous membranes have a structure that is designed for fluid flowthrough them. The fluid can be either a liquid or a gas, and generallythe pore size of the membrane must be at least several times the meanfree path of the fluid to achieve the desired flux. The pore size rangefor microporous membranes is generally from about 10 nanometers toseveral microns, with an average pore size less than about 1 micrometer.Such membranes are generally opaque because the pore size and polymermatrix are of sufficient size to scatter visible light. The term“microporous membrane” as used, is inclusive of other descriptions usedin the scientific and patent literature such as “microporous films”,“microporous sheets”, and “microporous webs”. The microporous membranescan also exhibit free-standing properties, and have interconnected poresthat extend throughout the membrane. “Free-standing” refers to amembrane having sufficient mechanical properties that permitmanipulation such as winding and unwinding in sheet form for use in anenergy storage device assembly.

Microporous membranes have been utilized in a wide variety ofapplications such as filtration, breathable films for garment or medicalgown applications, battery separators, synthetic printing sheets, and assurgical dressings. In some cases, the microporous membranes arelaminated to other articles (e.g., a non-woven) to impart additionalfunctionality (e.g., tear resistance, oxidation resistance). Themicroporous membrane may also undergo machine-direction ortransverse-direction stretching as part of the manufacturing process orin a secondary step.

The manufacture of microporous membranes is often accomplished bythermally induced phase separation. In this process, a homogeneousmixture is formed by melt blending the polymer with a thermally stableplasticizer (e.g., a paraffin oil) at elevated temperature, and thencasting or extruding into a non-porous film or object which is cooled toinduce phase separation of the polymer and plasticizer often as a resultof polymer re-crystallization. The plasticizer is then removed bysolvent extraction and drying to form a microporous membrane. In orderto facilitate the separation and recycling of solvent and plasticizer,it is important that their initial boiling points be greater than about50° C. apart (e.g., for efficient separation during distillation).

DETAILED DESCRIPTION

Battery separators are commonly manufactured using a thermally inducedphase separation process, followed by extraction of the thermally stableplasticizer with hexane, trichloroethylene, methylene chloride, or othersolvents. Government regulatory agencies continue to conduct riskevaluations on such solvents and have concerns regarding environmentaland worker exposures.

Most flooded lead acid batteries include polyethylene separators. Theterm “polyethylene separator” is a misnomer because these microporousseparators require large amounts of precipitated silica to besufficiently acid wettable. The volume fraction of precipitated silicaand its distribution in the separator generally controls its electrical(ionic) properties, while the volume fraction and orientation ofpolyethylene in the separator generally controls its mechanicalproperties. The porosity range for commercial polyethylene separatorsfor lead acid batteries is generally about 50-65%.

In the manufacture of Pb-acid separators, precipitated silica iscombined with a polyolefin, a plasticizer (i.e., process oil), andvarious minor ingredients to form a separator mixture that is extrudedat elevated temperature through a sheet die to form an oil-filled sheet.The oil-filled sheet is calendered to its desired thickness and profile,and the majority of the process oil is extracted with an organicsolvent; however, there is typically about 10-25% residual oil, morepreferably about 12-22% process oil left in the final separator. Anaphthenic process oil is preferred and the residual oil is intended toincrease the oxidation resistance of the Pb-acid battery separator.Hexane and trichloroethylene have been the two most common solvents usedin Pb-acid separator manufacturing. The solvent-laden sheet is thendried to form a microporous polyolefin separator and is slit into anappropriate width for a specific battery design.

The polyethylene separator is delivered in roll form to lead acidbattery manufacturers where the separator is fed to a machine that forms“envelopes” by cutting the separator material and sealing its edges suchthat an electrode can be inserted to form an electrode package. Theelectrode packages are stacked such that the separator acts as aphysical spacer and an electronic insulator between positive andnegative electrodes. Sulfuric acid is then introduced into the assembledbattery to facilitate ionic conduction between the electrodes.

The primary purposes of the polyolefin contained in the separator are to(1) provide mechanical integrity to the polymer matrix so that theseparator can be enveloped at high speeds and (2) to prevent grid wirepuncture during battery assembly or operation. Thus, the hydrophobicpolyolefin preferably has a molecular weight that provides sufficientmolecular chain entanglement to form a microporous web with highpuncture resistance. The primary purpose of the hydrophilic silica is toincrease the acid wettability of the separator web, thereby lowering theelectrical resistivity of the separator. In the absence of silica, thesulfuric acid would not wet the hydrophobic web and ion transport wouldnot occur, resulting in an inoperative battery. Consequently, the silicacomponent of the separator typically accounts for between about 55% andabout 80% by weight of the separator, i.e., the separator has asilica-to-polyethylene weight ratio of between about 2.0:1 and about3.5:1.

The manufacture of microporous membranes for synthetic printingapplications is exemplified by Schwarz et al in U.S. Pat. No. 5,196,262.In this case, the polymer matrix constitutes a blend of ultrahighmolecular weight polyethylene (UHMWPE) having an intrinsic viscositygreater than about 10 dl/g and lower molecular weight polyethylene witha melt flow index less than about 50 g/10 min (ASTM D 1238-86condition). These polymers are combined with a high percentage of finelydivided, water-insoluble siliceous filler, other minor ingredients, anda processing plasticizer to form a mixture that is subsequently extrudedinto a sheet from which the majority of the plasticizer is extractedwith a solvent. The residual plasticizer is less than about 3 wt %, morepreferably less than about 1%, of the synthetic printing sheet. Examplesof suitable organic extraction liquids include trichloroethylene,perchloroethylene, methylene chloride, hexane, heptane, and toluene. Theresultant microporous membranes are sold by PPG Industries under theTeslin® trademark.

Separators designed for Li-ion, Li-metal, or rechargeable Li-metalbattery systems are commonly manufactured using a thermally inducedphase separation process. In this case, various grades of polyethyleneranging in molecular weight from about 0.5 million g/mol to about 10million g/mol are combined with a plasticizer and then extruded througha sheet die or annular die to form an oil filled sheet. The oil-filledsheet is often biaxially oriented to decrease its thickness and improvemechanical properties in both the machine- and transverse-directions.Next, the biaxially-oriented sheet is most often passed through anextraction bath of methylene chloride to remove the plasticizer andsubsequently create pores upon evaporation of the solvent. The resultantbattery separator typically has thickness in the about 3-25 um rangewith porosity between about 40-65%, and less than about 1% residualplasticizer.

In the case of chlorinated solvents such as trichloroethylene (TCE), ithas been shown that it can absorb into the amorphous regions of apolyethylene article and adsorb to the surface of polyethylene powder.(See e.g., Shuai Xie et al., Very Low Concentration Adsorption Isothermsof Trichloroethylene on Common Building Materials, Building andEnvironment, Vol. 179, 2020 Jul. 15.) The same mechanisms can apply tobattery separators, where the chlorinated solvent can remain within thematerial even after passing through a drying oven at elevatedtemperature. In some cases, the chlorinated solvent can remain withinany residual plasticizer or oil that was used to extrude the separatorprecursor.

In response to ongoing environmental pressures and health concernsrelated to organic solvents such as trichloroethylene, methylenechloride, and hexane, there is a need for a new sustainable approach tothe manufacture of microporous membranes that can meet customerperformance requirements from a process that minimizes worker exposureto potentially harmful chemicals and efficiently recycles the extractionsolvent and plasticizer in a closed loop. The components of themicroporous membranes disclosed herein, including the solvents andplasticizers used in the manufacture thereof, can be environmentallyfriendly and safe for handling. The solvents can also be food-grade,minimizing health concerns with the handling thereof.

In the manufacture of microporous membranes using thermally inducedphase separation, the main considerations for solvent selection includephysical properties, chemical properties, equipment compatibility,safety, recyclability, cost, and the ability to achieve the desiredproduct characteristics (e.g., pore size distribution). A summary ofsolvent selection criteria are shown in Table I below:

TABLE I 1. Physical Properties Boiling point Flash point Vapor pressureDensity Viscosity Surface tension Heat of vaporization 2. ChemicalProperties Oil solvency Water solubility Reactivity Corrosiveness 3.Process Compatibility Equipment design & engineering Tooling Throughputrate Solvent recovery Vapor recovery 4. Product CharacteristicsMechanical properties Electrical (ionic) resistance Oxidation resistanceElectrochemical compatibility Cost

It is difficult to identify a single solvent, particularly one that isnon-flammable, that can meet all of the selection criteria while alsominimizing health and environmental risks. With continued pressure fromREACH and EPA to eliminate chlorinated solvents such astrichloroethylene and methylene chloride as extraction solvents for theproduction of microporous membranes used as battery separators, a newapproach is required. While certain azeotropes have been proposed assubstitutes for trichloroethylene and methylene chloride, they usuallycontain trans-dichloroethylene (i.e., a chlorinated solvent) combinedwith various fluoro-compounds which fall under the category of perfluoroalkyl substances (PFAS). The latter are under increased scrutiny becauseof their environmental persistence (i.e., they do not degrade) andassociated health risks.

In terms of physical and chemical properties of the solvent, it isimportant to consider them in relation to those of the plasticizer. Keyconsiderations include the flashpoint, boiling point range, and anilinepoint of the solvent and plasticizer. For instance, the flashpoint ofthe plasticizer can be important for extrusion. In particular, theflashpoint of the plasticizer should be higher than the extrusiontemperature, which is generally greater than about 215° C. during themanufacture of polyolefin battery separators. Thus, in some embodiments,the plasticizer has a flashpoint above about 215° C. In anotherembodiment, the plasticizer has a flashpoint above about 145° C. (e.g.,such as from about 145° C. to about 350° C., or from about 145° C. toabout 300° C.). For the extraction solvent, it is prudent that theflashpoint be above room temperature (greater than about 23° C.) so thatsufficient thermal energy can be utilized to facilitate evaporation andpore formation in the membrane, while mitigating the risk of fire. Thus,in some embodiments, the flashpoint of the extraction solvent is greaterthan about 23° C. In further embodiments, the flashpoint of theextraction solvent is greater than about 38° C. to satisfy certain codeand/or fire code requirements (e.g., such as the NFPA 30 Flammable andCombustible Liquids Code, the NFPA 1 Fire Code; and/or the InternationalFire Code as recognized in 2022-2023).

Because vapor pressure and volatility of a solvent generally correlatewith the boiling point, it is important that a temperature range beselected to minimize potential worker exposure while maximizing theability to separate the solvent from the plasticizer with the lowestpossible amount of energy. In addition, in some embodiments, it isimportant that the solvent has an initial boiling point that is at leastabout 50° C. lower than the flashpoint of the plasticizer to more easilyachieve separation through distillation. In additional embodiments, thesolvent has an initial boiling point that is at least about 85° C. lowerthan the flashpoint of the plasticizer. It will be appreciated that theinitial boiling point of the plasticizer will always be higher than itsflashpoint (to the extent the plasticizer has a flashpoint). Thus,stated another way, with respect to initial boiling points, the solventhas an initial boiling point that is at least about 50° C., or at leastabout 85° C., lower than the initial boiling point of the plasticizer.The difference in initial boiling points and/or flashpoints between thesolvent and the plasticizer is helpful for efficient distillation andseparation of the components such that one or both components can berecycled and reused in the manufacturing process, as further describedbelow in relation to FIGS. 1 and 2 . In some instances, it is preferableto have the difference in initial boiling points and/or flashpointsbetween the solvent and the plasticizer as great as possible.

Next, the aniline point is relatively simple test to measure thesolvency power of a hydrocarbon. Aniline is a simple aromatic amine withan amino group attached to a benzene ring. The aniline point is thelowest temperature at which a 1:1 mixture of a solvent or plasticizerwith aniline remains a clear solution. A low value indicates a strongersolvency power, while a high value corresponds to weaker solvency. Insome embodiments, solvents having a lower aniline point may be used,such as in Pb-acid separators where residual plasticizer can be leftbehind and remain in the resulting separator. In other embodiments,solvents having either a lower or higher aniline point may be used, suchas in Li separators where all or substantially all of the plasticizer isremoved from the resulting separator.

In general, a higher aniline point means a relatively low level ofdissolved aromatics. Such a difference can clearly be observed forparaffinic versus naphthenic plasticizers as shown in the carbon typeanalysis (ASTM D2140). As the paraffinic carbons decrease in thedifferent plasticizers, and correspondingly, the naphthenic and aromaticcarbons increase, the aniline point also decreases. It should be notedthat the flashpoint follows a similar trend.

In the case of hydrocarbon solvents, the aniline point generallydecreases as the alkane chain length decreases or degree of aromaticityincreases. The above trends and relationships can be observed in TableII.

TABLE II Molecular Wt Evaporation or chemical Density BoilingDistillation Flashpoint Rate @ 25 C. Chemical Name description (g/cc) Pt(° C.) Range (° C.) (° C.) (n-BuAc = 200) Solvents Toluene 92 0.87

10.5 109-111 8.9 200 Xylene 106 0.859

40 135-144 31.6 77 Hexane 86 0.082 58.7 64-74 −18 705 Heptane 100 0.63498.4 95-98 −1 275 Decane 142 0.73 174 172-177 45 7 IsoPar-E C5-9Isoparaffin 0.723 — 115-140 4 170 IsoPar-G C10-11 Isoparaffin 0.746 —152-177 44 16 IsoPar-H C11-12 Isoparaffin 0.761 — 181-193 60 5.7IsoPar-L C11-13 Isoparaffin 0.754 — 185-198 66 4.4 Plasticizers ShellRicolia 430X Paraffinic base 0.828 — 422-589 258 nil Nynex Nypar 330Paraffinic base 0.871 — 395-610 262 nil H&R Pionier 2058 Paraffinic base0.875 — N/A 262 nil Calumet Cabot 930 Paraffinic base 0.875 — 380-539240.6 nil Liquid Paraffin (LP) Paraffinic base 0.825 — 280-350 150 nilCalumet Hydrocal 750 Naphthenic base 0.912 — N/A 225 nil Nynas Nytex 820Naphthenic base 0.917 — 340-535 226 nil Carbon Type Analysis (ASTMD2140) Cp Cn Ca Aniline (paraffinic (naphthenic (aromatic Chemical NamePt (° C.) carbons) carbons) carbons) Solvents Toluene 8 — — — Xylene 9 —— — Hexane 65 — — — Heptane 68 — — — Decane 77 — — — IsoPar-E 73 — — —IsoPar-G 77 — — — IsoPar-H 81 — — — IsoPar-L 81 — — — Plasticizers ShellRicolia 430X 132.8 >99 0 0 Nynas Nypar 330 126 70 29 <1 H&R Pionier 2058119 87 33 0 Calumet Cabot 930 315.8 65 35 0 Liquid Paraffin (LP)

102-110 >99 0 0 Calumet Hydrocal 750 97.5 54 36 19 Nynas Nytex 820 83.949 40 11

indicates data missing or illegible when filed

In some instances, a larger difference in the aniline points of thesolvent and plasticizer results in faster extraction; however, a bespokesolvent is required to meet the other criteria outlined above forefficient recycling in a closed loop process. In some embodiments, theaniline point of the plasticizer is between about 70° C. and about 140°C., between about 75° C. and about 135° C., or between about 80° C. andabout 130° C. In another embodiment, the aniline point of theplasticizer is between about 35° C. and about 140° C. Surprisingly, wehave found that IsoPar-G can meet the requirements for next generationsolvent extraction and recovery processes for the manufacture ofmicroporous membranes using plasticizers that include naphthenic,paraffinic, and white mineral oils. Other solvents that meet thecriteria outlined above, including both solvents identified in Table IIand solvents not identified in Table II can also be used.

In some embodiments, the solvent is used to extract the plasticizerafter the non-porous film is stretched or biaxially oriented. In furtherembodiments, the resultant microporous polyolefin membrane is furtherstretched and/or biaxially oriented after extracting the plasticizer.The resultant microporous polyolefin membrane is also entirely free ofany halogen-containing compounds (such as residual halogen-containingcompounds) because the extraction solvent is halogen-free. This is incontrast to microporous polyolefin membranes manufactured with ahalogen-containing extraction solvent such as methylene chloride ortrichloroethylene, which can include trace amounts of residualhalogen-containing compounds. It should be recognized that trace amountsof residual halogen-containing compounds can potentially cause corrosionor other problems in the performance of rechargeable lithium-ion orlithium metal-based batteries.

The microporous polyolefin membranes disclosed herein can be made with avariety of polylefins. Exemplary polyolefins that can be used include,but are not limited to, various grades of polyethylenes, such aspolyethylenes ranging in molecular weight from about 0.5 million g/molto about 10 million g/mol (e.g., including ultrahigh molecular weightpolyethylene (UHMWPE), very high molecular weight polyethyelene(VHMWPE), high molecular weight, high-density polyethylene (HMW-HOPE),and mixtures thereof), polypropylenes, polymethylpentenes, and mixturesthereof. In particular embodiments, the polyolefin includes apolyethylene having a molecular weight of about 0.5 million g/mol orgreater, or about 0.6 million g/mol or greater.

The thickness of the microporous polyolefin membranes can vary. In someembodiments, the microporous polyolefin membrane comprises a backwebthickness of about 25 microns or less, or about 20 microns or less. Incertain embodiments, the microporous polyolefin membrane comprises abackweb thickness of between about 5 and about 25 microns, or betweenabout 5 and about 20 microns. Microporous polyolefin membranes having agreater thickness can also be made, such as those having a backwebthickness of between about 150 microns and about 300 microns, etc. Asused herein, backweb thickness can refer to the thickness of themicroporous polyolefin membrane that does not include the height of anyribs or surface protrusions.

The microporous polyolefin membranes can also optionally include afiller. Exemplary fillers that can be used include inorganic oxides,carbonates, or hydroxides, such as, for example, alumina, silica,zirconia, titania, mica, boehmite, magnesium hydroxide, calciumcarbonate, hydrotalcites, and mixtures thereof. The filler can bedistributed throughout the microporous polyolefin membrane. In someembodiments, the filler is uniformly distributed throughout themicroporous polyolefin membrane. In other embodiments, the filler is notuniformly distributed.

The microporous polyolefin membranes can also optionally be annealed orheat stabilized as part of the manufacturing process, after theextraction process and prior to being wound into a roll for later use.

The following examples are illustrative in nature and not intended to belimited in any way.

Example 1

UHMWPE (Celanese GUR 4150), precipitated silica (PPG WB-2085), and minoringredients (antioxidant, plasticizer, and carbon black) were combinedin a horizontal mixer and blended with low speed agitation to form ahomogeneous mix. Next, a hot naphthenic plasticizer (ENTEK 800 oil;Calumet) was sprayed onto the dry ingredients. This mix contained ˜58wt. % oil and was then fed to a 96 mm counter-rotating twin screwextruder (ENTEK Manufacturing Inc) operating at a melt temperature of˜215° C. Additional process oil was added in-line at the throat of theextruder to give a final oil content of ˜65 wt %. The resultant mass waspassed through a sheet die into a calendar and embossed with a ribpattern and a thickness of ˜200-300 um. After passing over two coolingrolls, the oil-filled sheet was collected for extraction.

A ˜160 mm×˜160 mm oil-filled sample was placed in beaker containingIsoPar-G and was extracted with agitation for 10 minutes at roomtemperature. The solvent laden sample was then dried in a circulatingoven overnight at 105° C. The resultant separator was porous, had goodmechanical properties, and contained 16 wt % residual oil (i.e.,plasticizer). As the solvent was free of halogen-containing compounds,the resultant separator was also free of residual halogen-containingcompounds.

Example 2

A naphthenic plasticizer (140 kg; ENTEK 800 oil; Calumet) was dispensedinto a Ross mixer where it was stirred and degassed. Next, the followingwere added and mixed with the oil:

-   -   64 kg UHMWPE (Molecular weight ˜5 million g/mol)    -   32 kg VHMWPE (Molecular weight ˜1 million g/mol)    -   32 kg HMW-HDPE (Molecular weight ˜0.6 million g/mol)    -   1.2 kg Li Stearate    -   1.2 kg Anti-oxidant

The mixture was blended at ˜40° C. until a uniform 47 w/w % polymerslurry was formed. The polymer slurry was then pumped into a 73 mmdiameter, co-rotating twin screw extruder, while a melt temperature of˜215° C. was maintained. The extrudate passed through a melt pump thatfed a 257 mm diameter annular die having a 2.75 mm gap. The throughputthrough the die was 135 kg/hr, and the extrudate was inflated with airto produce a biaxially oriented, oil-filled film with a ˜2000 mmdiameter, which then passed through an upper nip at 20 m/min to collapsethe bubble and form a double layer, which was subsequently side-slitinto 2 individual layers.

The oil-filled layers then passed through an extractor in which IsoPar-Gflowed counter-current to the direction of the layers. The oil/IsoPar-Gmixture in the first zone of the extractor was pumped to a distillationunit and separated for re-use. The solvent-laden layers then passed intoan oven with air knives to evaporate off the IsoPar-G solvent with thevapor being sent to a carbon bed system for adsorption and subsequentrecovery with steam. Such an exemplary closed loop manufacturing methodis depicted in FIG. 1 . Another exemplary closed loop manufacturingmethod is depicted in FIG. 2 , which utilizes a vapor condenser systemrather than a carbon bed recovery system. Finally, the extracted layerswere stretched 1.5× in an MDO (Machine Direction Orientation) with 80°C. roll temperatures followed by 2.0× in a TDO (Transverse DirectionOrientation) operating at 128° C. MDO refers to orientation in themachine direction, or along the direction that the material is beingfed; while TDO refers to orientation along the transverse direction; oralong a direction that is 90 degrees to the machine direction.

The microporous layers then passed through a nip and were separated andthen wound as individual rolls on a dual turret winder. The resultantbattery separator was ˜20 um thick with an average Gurley airpermeability value of 150 secs/100 cc air. The residual plasticizer wasdetermined to be 0.6% based upon thermogravimetric analysis. As thesolvent was free of halogen-containing compounds, the resultantseparator was also free of residual halogen-containing compounds.

With reference to FIGS. 1 and 2 , a non-porous, plasticizer-filled filmformed from a cast or an extruded polymer-plasticizer mixture 20 ispassed through a countercurrent flow extractor 22. A solvent suppliedfrom a solvent storage tank 24 and flow controlled by a fluid valve 26flows into countercurrent flow extractor 22 in a direction opposite tothat of the film. Extractor 22 produces in a first internal zone aplasticizer solvent mixture, which is pumped to a distillation unit 28,where the plasticizer and solvent are separated for reuse. Distillationunit 28 produces a solvent condensate in a purified state. The purifiedsolvent is returned to a second internal zone of countercurrent flowextractor 22 for reuse in combination with solvent supplied from solventstorage tank 24. The solvent-laden film exits countercurrent flowextractor 22 and is passed into a heated dryer 30, which is a source ofheat equipped with air knives that evaporate off the solvent and therebyproduce a solvent vapor. A microporous membrane 32 emerges from heateddryer 30. The microporous membrane can be further stretched or annealedbefore being wound into roll form.

In the solvent recovery system embodiment of FIG. 1 , the solvent vaporproduced by operation of heated dryer 30 is recovered byadsorption-desorption with use of a carbon bed system 34. The solventvapor evaporates onto activated carbon, which adsorbs the solvent. Steamis then used to thermally desorb the solvent from the activated carbonfor delivery to storage tank 24.

In the solvent recovery system embodiment of FIG. 2 , the solvent vaporproduced by operation of heated dryer 30 is recovered by a vaporcondenser system 36. The solvent vapor enters vapor condenser system 36for extraction of the latent heat of vaporization from the solvent vaporto thereby cool and condense the solvent. The recovered solvent isdelivered to storage tank 24.

FIGS. 1 and 2 show an outlet of solvent storage tank 24 connectedthrough fluid valve 26 to countercurrent flow extractor 22. Thisconfiguration implements closed loop solvent recovery system embodimentsin which the recovered solvent washes over the plasticizer-filled filmto continue plasticizer removal from the sheet passing throughcountercurrent flow extractor 22.

In some cases, it may be advantageous to combine use of activated carbonand vapor condenser solvent recovery systems for efficient recovery andrecycling of the solvent. Skilled persons will appreciate thatsolvent/water liquid phase separation may be a necessary part of therecovery process where steam is utilized as a heat source.

Example 3

A naphthenic plasticizer (140 kg; ENTEK 800 oil; Calumet) was dispensedinto a Ross mixer where is was stirred and degassed. Next, the followingwere added and mixed with the oil:

-   -   64 kg UHMWPE (Molecular weight ˜5 million g/mol)    -   32 kg VHMWPE (Molecular weight ˜1 million g/mol)    -   32 kg HMW-HDPE (Molecular weight ˜0.6 million g/mol)    -   1.2 kg Li Stearate    -   1.2 kg Anti-oxidant

The mixture was blended at ˜40° C. until a uniform 47 w/w % polymerslurry was formed. The polymer slurry was then pumped into a 103 mmdiameter, co-rotating twin screw extruder, while a melt temperature of˜215° C. was maintained. Simultaneously, fumed alumina/HDPE/oil pelletswere fed into the extruder. The extrudate passed through a melt pumpthat fed a 257 mm diameter annular die having a 2.75 mm gap. Thethroughput through the die was 230 kg/hr, and the extrudate was inflatedwith air to produce a biaxially oriented, oil-filled film with a ˜2250mm diameter, which then passed through an upper nip at 20 m/min tocollapse the bubble and form a double layer, which was subsequentlyside-slit into 2 individual layers.

An individual oil-filled layer (˜44 um thick) was then restrained in ametal frame that was clamped together. A ˜200 mm×˜200 mm area of theoil-filled layer was then exposed to an excess quantity of IsoPar-G for˜15 mins at room temperature while the solvent was agitated. Thesolvent-laden, alumina-filled PE film was then dried in a circulatingair oven for ˜10 min at 105° C. The membrane was then removed from theframe and found to have a thickness of 12 um with an average Gurley airpermeability value of ˜50 secs/100 cc. The residual plasticizer was 0.8wt % as determined by thermogravimetric analysis. As the solvent wasfree of halogen-containing compounds, the membrane was also free ofresidual halogen-containing compounds.

Example 4

UHMWPE (37.4 kg, ˜5 Million g/mol), lithium stearate (0.43 kg),antioxidant (0.37 kg), and a naphthenic plasticizer (132.6 kg, ˜12 cP at100° C.; ENTEK 800 oil; Calumet) were blended together in a Ross mixerto form a 22 wt. % polymer slurry. This slurry was pumped into a twinscrew extruder at 170 kg/hr, while a melt temperature of >200° C. wasmaintained. The extrudate passed through a melt pump that fed a 257 mmdiameter annular die having a 2.75 mm gap. The extrudate was inflatedwith air to produce a biaxially oriented film with a 2000 mm diameter,which was subsequently passed through an upper nip at 14.5 m/minute tocollapse the bubble and form a double layer. The collapsed double layersheet was slit open on both edges and then conveyed through an extractortank filled with IsoPar-G to remove the plasticizer. Next, the extractedsheet was sequentially stretched in the machine direction (1.83×) andtransverse direction (2.9×) at 100° C. (˜1% relax), and then wound inroll form onto a cardboard core at 28.5 m/minute. The residualplasticizer was 0.4 wt % as determined by thermogravimetric analysis. Asthe solvent was free of halogen-containing compounds, the extractedsheet was also free of residual halogen-containing compounds.

Physical properties of the manufactured separator are shown in TableIII.

TABLE III Emveco Basis weight Gurley Name Thickness (μm) (g/m2) Porosity(%) (sec/100 c) Ex 4 11.5 3.7 67 46

Example 5

Polymer powders A thru C were individually weighed out into Aluminumpans and then placed on a tray in a glass vessel that was suspended inabout 0.5 liters of dichloromethylene (DCM). The vessel was then closedand sealed with a glass lid, and the polymer powders were allowed toadsorb/absorb the methylene chloride vapor for 24 hrs at roomtemperature (19° C.). The powder samples were then immediately weighedupon removal from the vessel, and the weight gain was calculated. Thesamples were then re-weighed after sitting in a fume hood for 30 mins.The data are shown in Table IV below.

TABLE IV Sample A B C PE Grade UHMWPE VHMWPE HMW-HDPE Approx MolecularWeight 5.0 1.0 0.6 (Millions g/mol) Al pan 2.208 2.2302 2.2206 Al pan +PE 18.7006 20.8746 23.1714 wt after 24 hrs DCM exposure 19.24 21.4724.35 in closed glass vessel DCM wt 0.54 0.60 1.18 % pick up 3.3 3.2 5.6wt after 24 hrs DCM exposure 18.7112 20.8874 23.2086 followed by 30 minunder hood DCM wt 0.0106 0.0128 0.0372 % pick up 0.0643 0.0687 0.1776

1. A free-standing microporous polyolefin membrane comprising: asemi-crystalline polymer matrix resulting from a phase separation of apolymer and a plasticizer having a flashpoint greater than about 215° C.and an aniline point between about 70-140° C.; the polymer matrixincluding polyethylene to provide mechanical integrity; interconnectedpores resulting from extraction of the plasticizer with a halogen-freesolvent and its subsequent evaporation; the halogen-free solvent havinga flashpoint greater than about 23° C. and an initial boiling point atleast about 50° C. lower than the flashpoint of the plasticizer.
 2. Thefree-standing microporous polyolefin membrane of claim 1, wherein thehalogen-free solvent comprises a flashpoint greater than about 38° C. 3.The free-standing microporous polyolefin membrane of claim 1, whereinthe polymer matrix is stretched or biaxially oriented.
 4. Thefree-standing microporous polyolefin membrane of claim 1, wherein thepolymer matrix is stretched or biaxially oriented prior to solventextraction of the plasticizer.
 5. The free-standing microporouspolyolefin membrane of claim 4, wherein the polymer matrix is furtherstretched or biaxially oriented after extraction of the plasticizer andevaporation of the solvent.
 6. The free-standing microporous polyolefinmembrane of claim 1, wherein the free-standing microporous polyolefinmembrane is annealed or heat stabilized prior to being wound.
 7. Thefree-standing microporous polyolefin membrane of claim 1, wherein thepolymer matrix further comprises a filler distributed throughout thepolymer matrix.
 8. The free-standing microporous polyolefin membrane ofclaim 7, wherein the filler comprises an inorganic oxide, carbonate,hydroxide, or mixtures thereof, or wherein the filler comprises alumina,silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calciumcarbonate, hydrotalcites, or mixtures thereof.
 9. The free-standingmicroporous polyolefin membrane of claim 1, wherein the polyolefincomprises a polyethylene having a molecular weight of at least about 0.5million g/mol. 10-11. (canceled)
 12. The free-standing microporouspolyolefin membrane of claim 1, wherein the free-standing microporouspolyolefin membrane contains about 10-25 wt % residual plasticizer, orabout 12-22 wt % residual plasticizer.
 13. (canceled)
 14. Thefree-standing microporous polyolefin membrane of claim 1, wherein thehalogen-free solvent comprises an initial boiling point at least about85° C. lower than the flashpoint of the plasticizer.
 15. Thefree-standing microporous polyolefin membrane of claim 1, wherein thefree-standing microporous polyolefin membrane is free ofhalogen-containing compounds.
 16. The free-standing microporouspolyolefin membrane of claim 1, wherein the halogen-free solvent is afood grade solvent.
 17. A method for the manufacture of a microporouspolyolefin membrane, comprising: melt blending a polyolefin and aplasticizer to form a mixture; casting or extruding the mixture into anon-porous sheet; cooling the non-porous sheet to induce phaseseparation of the polyolefin and plasticizer; extracting the plasticizerfrom the non-porous sheet with a halogen-free solvent and evaporatingthe solvent to form a microporous polyolefin membrane, wherein thehalogen-free solvent comprises a flashpoint above about 23° C. with aninitial boiling point at least about 50° C. lower than a flashpoint ofthe plasticizer.
 18. (canceled)
 19. The method of claim 17, wherein theplasticizer comprises a flashpoint greater than about 215° C. and ananiline point between about 70-140° C.
 20. The method of claim 17,wherein the halogen-free solvent comprises an initial boiling point atleast about 85° C. lower than the flashpoint of the plasticizer.
 21. Themethod of claim 17, comprising a closed-loop, solvent extraction,drying, and carbon bed recovery system. 22-30. (canceled)
 31. A solventladen sheet, comprising: a cast or extruded polyolefin sheet, the castor extruded polyolefin sheet resulting from a cast or extruded mixtureof the polyolefin and a plasticizer having a flashpoint greater thanabout 215° C. and an aniline point between about 70-140° C.; and ahalogen-free solvent loaded within the polyolefin sheet.
 32. (canceled)33. The solvent laden sheet of claim 31, wherein the halogen-freesolvent comprises a flashpoint greater than about 23° C. and an initialboiling point at least about 50° C. lower than the flashpoint of theplasticizer.
 34. The solvent laden sheet of claim 33, wherein thehalogen-free solvent comprises an initial boiling point at least about85° C. lower than the flashpoint of the plasticizer. 35-37. (canceled)